Ferrofluidic seals are commonly utilized to provide a hermetic seal against gas and other contaminants in applications where a rotary shaft must be sealed. Ferrofluidic seals have been utilized in computer magnetic disc storage units as a barrier between the motor area and the disc area for preventing contaminants from reaching the disc area, for robotic actuators designed for use in ultra-pure vacuum processing of semi-conductor wafers, for sealing rotary anodes in high vacuum environments, and for pumps in refineries and chemical plants.
Ferrofluidic seals can be utilized to seal any component which is in stationary or rotational relation with another component. Such seals are normally installed to remain stationary about rotating shafts, but can be installed to seal a stationary shaft about which a hub rotates. The seals operate generally through the employment of ferrofluid in a gap between a rotating shaft and stationary seal surfaces, and include an annular magnet for providing a desired magnetic flux path which retains and concentrates the ferrofluid in a seal-tight liquid O-ring in the gap. Ferrofluidic seals typically include a permanent annular ring magnet polarized axially, and a pair of magnetically-permeable annular pole pieces which sandwich the magnet, so that inner peripheral edges of the pole pieces extend toward and form a close, non-contacting gap with the exterior shaft surface.
Ferrofluidic seals have been constructed and arranged to address a variety of sealing needs. A single-stage ferrofluidic seal can be created by placing a single annular pole piece in close proximity with and surrounding a magnetically-permeable shaft and in magnetic communication with a single magnet. Ferrofluid is retained in the pole-piece/shaft gap by the magnetic field created by the magnet, which field follows a magnetic circuit containing the magnet, pole-piece, the gap, and the shaft. A second annular pole piece also in close proximity with the shaft and in magnetic communication with the other pole of the magnet can advantageously be employed in a single-stage seal. The gap between this second pole-piece and the shaft generally contains no ferrofluid but enhances the magnetic flux across the gap within which ferrofluid is retained in the single-stage seal, thereby increasing the pressure capacity of the seal.
Alternatively, the gap between the second pole-piece and the shaft may contain ferrofluid, creating a two-stage seal. U.S. Pat. No. 5,018,751, issued May 28, 1991, and U.S. Pat. No. 4,506,895, issued Mar. 26, 1985, and assigned to the same assignee of the present invention, describe the above-noted two-stage ferrofluid seals.
In addition, a ferrofluid seal may comprise any number of stages, that is, the seal may comprise a plurality of discrete pole-pieces, or a pole-piece (or pole-pieces) may include a plurality of ridges and grooves, each ridge in close proximity with the shaft and defining an annular gap between the pole-piece and the shaft. Ferrofluid is retained in several or all of these gaps to form a multi-stage ferrofluidic seal.
A number of embodiments comprising a variety of arrangements and designs exist with respect to ferrofluidic seal pole-pieces and with respect to the relationship between pole-piece(s), shaft-support bearings, and other components of a ferrofluidic sealing system. For example, ridges and grooves may be formed in a shaft, rather than in a pole-piece or pole-pieces, to define a single-stage or multi-stage ferrofluidic seal. A pole-piece (or one or more of several ridges in a pole-piece) may be tapered, or may be of a particular width or have a concentric radius different from that of other ridges or pole-pieces. Geometric variances such as these allow tailoring of particular properties of an overall ferrofluid seal arrangement such as the longevity of a particular gap with respect to ferrofluid retention, advantageous heat dissipation, the prevention of ferrofluid splashing at high shaft or hub rotation speeds, the selective retention of ferrofluid in one or more annular gaps, and other properties.
It is important, for proper-operation of a ferrofluidic seal, to accurately mount the pole piece or pole pieces concentrically with the shaft. Inaccurate centering of the pole piece(s) about the shaft will result in non-uniform width of the resultant annular gap between the pole piece(s) and the shaft. Generally, it is the pole piece or pole pieces which are in closest proximity with a shaft and for which centering is most critical. However, other components which create a ferrofluid seal may be in close proximity with the shaft as well. For example, an annular magnet may be sandwiched by pole-pieces, the magnet and pole pieces all having identical inner radii. It is to be understood that it is critical to center any component of a ferrofluid seal arrangement which is in close (especially that component which is in closest) proximity with the shaft. Herein below, "seal" will refer to components of a ferrofluid seal which are in close proximity with the shaft and with respect to which centering is critical.
If a seal is not mounted concentrically with a shaft, the magnetic field will not follow a symmetric path about the shaft, but magnetic flux will be enhanced at the narrowest portion of the gap and diminished at the widest portion. This uneven distribution may also prevent the fluid from flowing evenly during operation and may either cause such fluid to heat up and evaporate, or splash out of the gap area. Thus, when the shaft is stationary or rotating slowly relative to the housing, ferrofluid may not be evenly distributed within the annular gap, but may be drawn to the narrowest portion of the gap leaving the widest portion with insufficient fluid to maintain adequate sealing strength. Such sealing strength inadequacy can lead to seal "bursting" at a lower threshold differential across the seal than would normally occur. U.S. Pat. No. 4,407,518, issued Oct. 4, 1983 and assigned to the same assignee of the present invention graphically illustrates ferrofluid seal bursting.
The nature of ferrofluidic seals is that they will "self-heal" to some extent after such bursting. That is, ferrofluid ejected from the seal during bursting which is not displaced beyond a particular distance from the seal within the reach of the magnetic field of the seal may be drawn back into the active seal area. However, net loss of ferrofluid from the seal generally does occur with each burst, thus repeated occurrences of seal overpressurization can result in reduction in seal pressure capacity.
In addition, dynamic eccentricity is inherent in any rotating shaft, thus the contribution from shaft eccentricity due to inaccurately centered ferrofluidic seals must be minimized. A very high speed rotary shaft or a shaft which is relatively unevenly supported by bearings (for example to allow long shaft overhang) may exhibit excessive dynamic eccentricity. Also, a shaft supported by aged bearings may exhibit an unacceptable degree of dynamic eccentricity. Such dynamic eccentricity may create, among others, two notable problems. First, the shaft may come into contact with a portion of the seal and deflect the seal or cause a magnetic short circuit, thereby negatively affecting its operation. The shaft may also be damaged by such contact with the seal. Second, when the shaft is most eccentrically displaced during rotation, the resultant uneven annular gap forms a ferrofluid seal which is weakened at the widest portion of the gap. Thus, bursting as described above may occur at the weakened portion of the seal. Therefore, it is critical to the operation of ferrofluidic seals, and desirable for avoiding shaft damage, for seals to be accurately centered about shafts.
Customarily, accurate centering of a seal about a shaft is effected by accurately piloting (registering) the shaft-support bearings and the seal in a common housing. The accuracy of this method relies upon the accuracy of shaft centering in the housing and the accuracy of the surface against which the seal pilots. Construction of a shaft/seal arrangement in such a way is advantageously carried out at one location to assure overall accuracy.
In some cases, however, the seal cannot be contained within or piloted against a housing in which the shaft is accurately mounted, the seal may not be mountable close to a bearing in precise relationship with the shaft, the housing upon which the seal is to be mounted may not be fitted with a recess for accommodating the seal, it may be inconvenient or impossible to verify the concentricity of an existing recess with the shaft, or the end of the shaft may not be accessible for sliding the seal over the shaft. It is not uncommon to encounter one of these situations when mounting a ferrofluid seal onto an existing apparatus which requires additional sealing capacity, when repairing or replacing components in an apparatus utilizing ferrofluidic seals, when replacing ferrofluidic seals, or when performing other activities in the "field", that is, away from precision machining and assembly equipment. A notable example follows. It is increasingly important, in oil refinery pumps, to contain any volatile fumes which may escape as a result of the leakage of volatile liquids. The operation of a refinery pump typically involves a shaft driven by a motor and supported by precision bearings in a first housing, the shaft passing through a space which serves as a service access and through a mechanical face seal into a second housing, or pump area, in which an impeller is driven by the shaft to propel a liquid such as gasoline. The mechanical face seal is not a bearing and does not support the shaft with any degree of positional precision. The mechanical face seal relies on minute leakage across the seal faces for lubrication. State of the art low emission seals actually operate with vapor at the faces rather than a liquid. Thus, a volatile vapor is slowly but continuously released into the access area and then into the atmosphere. It is advantageous to seal a portion of the access area which immediately surrounds the mechanical face seal to contain fumes from the volatile vapor, and ferrofluidic seals are ideal for this purpose. However, there is no convenient means of accurately mounting a ferrofluidic seal on a mechanical face seal enclosure, and mechanical face seals require relatively frequent service, thus the removal and replacement of ferrofluidic seals is especially important in this application. Indeed, it is desirable to mount ferrofluid seals of this type in existing refinery pumping stations and in many other existing sources of noxious fumes such as chemical plants, thus techniques applicable for "field" work are especially desirable.
One past approach for centering a seal about a shaft in applications such as those described above involves sliding the seal over and down the shaft to the housing, thereafter sliding a tapered tubular sleeve over and down the shaft, tapered end first, and forcing the sleeve between the seal and the shaft. The sleeve, being tapered, is wedged concentrically between the seal and shaft, thereby centering the seal about the shaft. The seal may then be permanently fastened to the stator of the equipment. Finally, the tubular sleeve is removed.
Another past approach involves wrapping thin metal shim stock around the shaft before sliding the seal over the shaft. The shim stock lies concentrically and evenly about the shaft for centering the seal which is slid over the shim stock. After affixing the seal to the existing equipment, the shim stock is removed.
These past approaches for centering ferrofluidic seals have associated drawbacks and problems. In particular, the approaches involving the tubular sleeve and the shim stock are burdensome and time consuming. In addition, it is important that the tubular sleeve or shim stock be placed precisely in position between the seal and the shaft in order to create an accurate concentric air gap, before fastening the seal to a stationary stator. The precise positioning of the tubular sleeve and shim stock is not easily achievable. As such, a difficult and tedious procedure is required centering the seal. Additionally these centering methods are not compatible with pre-assembled cartridge seal designs.
Additionally, no method currently exists for retaining ferrofluid, expelled from a ferrofluidic seal by seal bursting due to overpressurization, within close proximity of the seal such that the expelled ferrofluid will be drawn back into the active seal area by the magnetic field.
Accordingly, a general purpose of the present invention is to provide a self-activating mechanism for centering a ferrofluidic seal about a shaft, which mechanism is convenient and rapidly employed, to provide a mechanism for centering a ferrofluidic seal, which seal is retrofitted about a shaft having neither end accessible, to provide a mechanism for centering a ferrofluidic seal, which mechanism may be left permanently in place within the seal after the seal is installed, to provide a mechanism for centering a ferrofluidic seal, which mechanism may be easily removed after the seal is installed, and to provide a means and mechanism for retaining ferrofluid, expelled from the seal via bursting due to overpressurization, in close proximity to the seal such that a substantially large portion of the expelled ferrofluid is retained within reach of the magnetic field of the seal so as to be drawn back into the active seal region such that minimal net loss of ferrofluid occurs during bursting, thus minimizing the reduction in pressure capacity of the seal due to such bursting.